1. Field of the Invention
The present invention relates to an installation and a process for continuously charging solid material into a reactor and heating said material by heat exchange with the hot gases emitted from the reactor such as the smokes produced at the time of manufacture of steel from pig iron in a converter.
2. Description of the Prior Art
It is known that, in the production of steel from pig iron, a large quantity of hot combustible gases is emitted from the mouth of the converter.
In the past, the heat energy of this combustible gas was lost in nearly all cases.
With modern methods of conversion of pig iron by the use of oxygen, it has now become possible to recover a more or less substantial fraction of this energy.
Three modes of recovery of heat energy are essentially distinguished:
the so-called recuperation boilers which generate steam;
collection of gases by the so-called combustionless process; in this case the recovered gases are utilized as conventional fuel for various requirements;
heating of steel scrap by these gases or the resultant smokes prior to charging into the converter.
The distinctive features of these three modes of recovery are as follows:
when making use of recuperation boilers, an average of 50% of the latent and sensible heat is recovered but the production of steam follows the conversion cycle and is therefore non-continuous;
when collecting gases by the combustionless process, only the latent heat of gases is usually recovered in the case of non-continuous production of gas as mentioned above, thus entailing the need for storage in a gasometer; gas recovery remains at a medium level (about 50% of the total energy contained in the gases) on account of the precautions which have to be taken in order to guard against explosions;
when heating steel scrap and charging this material into the converter; the quantity of steel scrap required in order to produce one ton of steel can be increased whilst the quantity of pig iron can be correlatively reduced.
It is known that the production of pig iron calls for a very large amount of energy (4500 to 6000 thermies, namely 18.8 to 25 gigajoules per ton of pig iron).
To give a clear idea, the production of one ton of steel from high-phosphorus pig iron in accordance with modern methods requires on an average 750 kilograms of pig iron and 330 kilograms of cold steel scrap. At the time of this conversion, the bath emits gases which contain approximately 220 thermies (920 megajoules) in the form of latent and sensible heat per ton of pig iron. A 50% recovery therefore corresponds to 110 thermies (460 megajoules) recovered per ton of pig iron.
Recovery with heating of steel scrap permits the following results: when recovering 90% of the heat energy of the smokes, which is a reasonable expectation, the material and thermal balance shows that, in order to produce one ton of steel, it is necessary to consume 514 kilograms of pig iron and 530 kilograms of steel scrap or swarf. There are consequently consumed 750-514=236 kilograms less pig iron and 530-330=200 kilograms more steel scrap. The gain due to the non-manufactured pig iron is 0.236.times.4500=1062 thermies (4.44 gigajoules) as a minimum reduction per ton of steel produced. This figure is consequently much higher than the 220.times.0.750.times.0.9=148 thermies (621 megajoules) per ton of steel recovered in the smokes. Furthermore, by utilizing the heat contained in the smokes to an even greater extent, purification is thus achieved. Smoke purification is in fact made compulsory in practically all areas with a view to protecting the environment.
Among the known methods for heating steel scrap, the following are worthy of mention:
those which consist of surface licking or moderate penetration of steel scrap by hot smokes (reference may be made in this connection to French Pat. Nos. 1,312,160, 1,331,339, 1,387,077; German Pat. No. 1,508,292 and U.S. Pat. Nos. 3,425,676 and 3,533,612); the major disadvantage of the devices or processes described in these patents lies in a very incomplete recovery of the energy of gases unless provision is made for inordinate heat-exchange path lengths;
those which consist of intimate penetration of smoke through a mass of steel scrap which completely obstructs the smoke circulation system; in these methods, the scrap is always motionless or largely motionless during a production cycle (see French Pat. Nos. 1,138,829, 1,548,324, 1,589,630, 1,401,905 and German Pat. No. 1,508,297); in these devices, the degree of recovery is limited by the fact that the smokes leaving the system become progressively hotter during the heat exchange process;
those which make use of a rotary tunnel kiln (see French Pat. Nos. 1,138,829 and 1,434,287); by means of these recuperators, energy is recovered by radiation with fairly good efficiency in the hot portions but with low efficiency in the cold portions and inadequately by convection along the entire tunnel, thus entailing the need for excessive lengths if it is sought to utilize the entire thermal energy of the gases;
those which employ recuperators on grates through layers of steel scrap of moderate depth (see British Pat. No. 933,353 and U.S. Pat. No. 3,301,662); the crossed flow streams which are necessary in this case together with the fact that the smokes are discharged at very high temperature after passing through very hot scrap fail to permit total recovery of the heat energy of smokes.
In accordance with the present invention, the Applicant has established the fact that, in order to achieve a high degree of recovery of the heat energy of gases, the following conditions must be satisfied:
total collection without any leakage of gases and especially hot gases at any location;
prevention of parasitic admissions of cold air;
stoichiometric combustion of the gases;
penetration of the entire quantity of steel scrap by the entire quantity of smoke without any possible shortcircuit;
the need for a sufficient heat-exchange surface area (either by conditioning the steel scrap or by increasing the quantity of scrap);
the achievement of equilibrium of thermal flux values throughout the charge in order to prevent preferential heat-exchange processes or zones of insufficient heat exchange;
the need for continuous charging of cold steel scrap into the gas delivery circuit;
circulation of gases as far as possible with parallel countercurrent flow streams;
minimum heat loss through the walls, thus making it necessary among other things to ensure that the external surface area of the installation is as small as possible.